The synthetic discriminant function (SDF) has evolved over many years from the first purely linear encoding to the first practically usable minimum average correlation energy (MACE). It is based on the idea of using a linear or nonlinear combination of reference images of an object to create a composite image filter of the object. The composite filter can be thought of as a type of combination of the target object reference images from multiple positions and scales. The resulting encoding, regardless of the specific encoding method used, produces the SDF filter, a complex-valued two dimensional filter which can be as large as the input image size.
LADAR (laser detection and ranging) works on the same principle as microwave RADAR in which either pulsed or continuous wave electromagnetic radiation is emitted toward and reflected from a potential target object. The reflected signal is received and analyzed to determine the distance between the LADAR and the reflecting objects. The short wavelength of the laser allows the beam to be concentrated in a very narrow field of view. The short wavelength also enables a high range resolution that allows the detailed structure of the target to be examined in depth. Imaging LADAR receivers are very similar to RADAR receivers in their use of ranged gating technology in pulsed systems or phase difference detection in continuous wave receivers. The LADAR reveals four properties of the reflected signal that can be used to differentiate man-made targets from the natural background. First, range data can be used to examine the surface geometry of the target object, including vibration effects; second, intensity data provides information about the nature of the surface material; third and fourth, polarimetric and velocity data, respectively, allow the man-made objects to be distinguished from the background.